The invention generally relates to data interchange in structured communications systems, in which calls are made by interchanging data packets. A particular field of application of the invention is that of systems implementing the ATM (Asychronous Transfer Mode) technique, in which the interchanged packets are of fixed size, and are referred to as "cells".
More precisely, the invention concerns routing cells (or more generally packets) in a private network, in particular in a network made up of a plurality of mutually distant sites interconnected via a public network.
Conventionally, a packet comprises at least two fields: a header and a data field. The data field contains data that is useful to the call. The header contains, in particular, routing data enabling the packet to be transferred from the transmitter to its destination via various nodes making up the communications network. The header has a predefined format so that it can be interpreted by all of the nodes of the network, and it must be as short as possible, so as to leave as much room as possible available for the data that is useful to the application.
Furthermore, it is known that private networks and public networks have different requirements in terms of routing data, if only because of the much greater number of calls handled by a public network.
The ATM technique provides a two-level hierarchical network organization, namely comprising virtual paths (VP) and virtual channels (VC). Generally, the paths are set up and managed semi-permanently, as are the transmission resources, under the control of mechanisms for administering the public network, whereas the channels are set up under the control of mechanisms for processing calls and connections.
In other words, the paths, which are identified in the header of each cell by a virtual path identifier (VPI) field are managed by the public network only, at distributors. The channels are identified by a virtual channel identifier (VCI) field enabling the cell switching to be managed.
In a private network, only the virtual channel concept is implemented. Therefore, it can be understood that the VPI field is unnecessary in the private network, but that it becomes necessary as soon as a call uses the public network.
A general object of the invention is to provide a method of optimizing use of the bits forming the header of a packet, in particular for routing the packet to its destination.
In this way, an object of the invention is to provide such a method that makes it possible to re-use, at private level, a header field that is provided to be used over the public network only, such as the VPI field of the ATM standard, while guaranteeing the continued existence of this field whenever it is necessary to transfer a packet to the public network.
In other words, an object of the invention is to provide such a method that enables the same field of the header of a packet to have two uses, depending on whether the packet is in a public network or in a private network, and that restores the field each time the packet passes from the private network to the public network, and vice versa.
In other words, an object of the invention is to broaden the possibilities for use of a packet header, without increasing the size of the header, or modifying its structure too significantly.
In this way, a particular object of the invention is to provide such a method that enables packets to be routed to a large number of stations. In particular, an object of the invention is to provide such a method that, in systems using the ATM technique, makes it possible to address a higher number of stations than known methods, whether using conventional routing via the VCI field, or the "self-routing" technique.
In accordance with the ATM standard, each cell entering a switching matrix of the network undergoes processing which consists inter alia in reading its header (more precisely the contents of the VCI field and/or the VPI field) so as to determine that direction in which it must be switched at the output.
Such information is associated with the number of the input multiplex via which the cell arrived so as to constitute the address of the translation memory of the matrix. The read data is constituted firstly by the number(s) of the multiplexes via which the cell is to be transmitted at the output, and secondly by the new VCI (or VPI) code(s). The first operation is referred to as "routing", and the second is referred to as "translation".
An ATM multiplex is designed to convey a large number of simultaneously-routed calls. These calls are routed via virtual connections identified by the VCI fields. On any given multiplex, all of the virtual connections that carry the same VCI belong to the same call.
Since the standard defines a VCI field of 16 bits, the maximum number of virtual connections routed simultaneously on the same multiplex is about 65,000, which corresponds to a translation memory size of 16.times.65,000 words of (a minimum of) 32 bits for an ATM matrix having 16 incoming multiplexes and 16 outgoing multiplexes.
In addition to the drawback of requiring a large-sized translation memory, that is therefore costly because of the silicon area required, that routing technique implements message interchange (marking) with a control processor during set-up and clearing-down of each connection. Such interchange is necessary with all of the switching matrices situated on the route to be travelled by the cells of the virtual connection to be set up. The number of switching layers that interconnect the terminals between which a call is to be set up may, in some cases, be large because this number depends on the size of the site, on the route followed, and on the topology of the network.
Use of large-sized (e.g. 16.times.16) matrices is quite compatible with these requirements insofar as:
a) the ratio between the required translation memory volume and the hardware taken up by the matrix (one or two cards) remains very advantageous; and PA1 b) the marking stages concern only a limited number of items of switching equipment. PA1 each of said packets comprising a header and a data field, said header containing at least two identifiers defining two access hierarchy levels as seen from said public network, namely a first identifier referred to as the "virtual channel identifier" (VCI) enabling switching of said packets to be controlled, and a second identifier referred to as the "virtual path identifier (VPI) enabling distribution of said packets to be controlled in said public network; PA1 in which method two tasks are assigned to said virtual path identifier, namely: PA1 in which method, each time a packet is transferred from one of said private sub-networks to said public network, routing data contained in said virtual path identifier is shifted to a reserved portion of said virtual channel identifier, and is replaced by distribution control data; PA1 and in which method, each time a packet is transferred from said public network to one of said private sub-networks, said distribution control data is erased, and said self-routing data contained in said reserved portion of said virtual channel identifier is shifted to said virtual path identifier. PA1 said virtual path identifier is constituted by 12 bits, the four most significant bits being forced to a predetermined value (in other words, only the eight least significant bits are used); and PA1 said virtual channel identifier is constituted by 16 bits, the eight most significant bits being reserved for storing said self-routing data. PA1 a plurality of switching nodes connected together in pairs via at least one data interchange medium, each of said nodes serving a respective concentrator, each concentrator managing at least one data processing terminal capable of transmitting and/or receiving data packets; and PA1 an interface module providing the connection between said private sub-network and said public network; PA1 a data packet is transmitted by a transmitter terminal to its associated concentrator, referred to as the "transmitter" concentrator, and on to a receiver terminal, said packet carrying a local VCI (VC-La) and a local VPI (VP-La) allocated by said transmitter terminal for the duration of a call; PA1 said local VCI is translated in said transmitter concentrator into a VCI (VC-Sa) allocated by said receiver terminal for the duration of a call; PA1 said local VPI is translated in said transmitter concentrator into a VPI (VP-Sa) allocated permanently to said receiver terminal; PA1 if said receiver terminal belongs to the same sub-network as said transmitter terminal: PA1 if said receiver terminal belongs to another sub-network: PA1 each switching node is associated with: PA1 and each matrix receiving a packet systematically selects one of said second outputs of the switching matrix if the identifier of the packet does not correspond to any of the identifiers contained in the memory, and one of said first outputs otherwise. PA1 and said identifier comprises two portions: PA1 so that, in each link node, the following steps are performed:
Unfortunately, as soon as various reasons (topology, modularity, maintenance, expandability, etc.) justify implementation of small-sized (or even single-component) switching equipment, the question of reducing the sizes of translation memories becomes relevant.
The self-routing technique is not governed by a standard but it is nevertheless widely used in widely differing forms. In principle, it consists in explicitly coding, in a field of a header of a cell, the list of the numbers of the links via which the cell is to be routed.
The drawback with that technique, which may be referred to as the "consumable label" technique, is that it requires a field having a length that is proportional to the number of switching layers via which the cell is to pass: it must therefore be reserved for small-sized networks.
By way of example, a field of 16 bits makes it possible to code the successive passes through only 4 16.times.16 matrices (4 bits for coding one output multiplex from 16). This drawback can however be overcome by means of various solutions.
A first solution consists in using the 24 bits represented by the VPI field and the VCI field together. That suffers from two drawbacks, namely it is necessary to provide a specific routing analysis logic circuit in each of the matrices, and it is impossible to superpose a second VCI or VPI routing level.
A second solution consists in encapsulating the 5-byte (standardized size) header with additional bytes, e.g. 3 bytes.
Another solution uses a compound approach: self-routing then translation (i.e. updating the self-routing label), then self-routing, etc.
The cell self-routing technique eliminates the translation memory marking stage, which becomes unnecessary. It also makes it possible if not to simulate the translation memories, at least to reduce their capacities significantly.
Another advantage of self-routing is that it offers, quite cheaply, the possibility of using back-up routes rapidly for transferring data in the event of local breakdowns or of traffic congestion. It is merely necessary to change the contents of the self-routing address in the transmitter terminal in order for the cells of the virtual connection to be forwarded over a different route that can by-pass the obstacle; naturally, after the self-routing address has been stored in the cell transmitter equipment.
Another object of the invention is to provide a method offering the advantages of self-routing, without suffering from the drawbacks thereof.
In particular, an object of the invention is to provide such a method that enables a network to be covered that has a size which is compatible with the requirements of private sites, while maintaining the structural integrity of the fields as defined in the standards, such as the VCI and VPI fields in ATM.
Another object of the invention is to provide such a method that is compatible with all types of network (single or double ring, bus, star, etc.) and with all combinations of networks. In particular, an object of the invention is to provide such a network that makes it simple to interconnect a plurality of sub-networks, either directly or via the public network.
Yet another object of the invention is to provide such a method that enables small-capacity translation memories to be used, thereby making it possible to integrate small-sized switching entities, e.g. in the form of ASIC-type components. Using small-sized switching components offers numerous advantages, such as modularity and mass-producibility, thereby making it possible to optimize manufacturing, installation, and maintenance costs, and to make operating costs linear, and the possibility of implementing widely differing network architectures: in particular, distribution networks can be built using centralized topologies but also geographically distributed topologies, e.g. loops, buses, or trees.
Another object of the invention is to provide such a method that does not require a specific self-routing mechanism, i.e. additional equipment, in the switching matrices. In other words, an object of the invention is to use matrices that comply with the standard, and therefore that are compatible with use in public network multiplexing and switching equipment.